1. Field of the Invention
The present invention relates to a liquid discharge head for discharging liquid, and more particularly, it relates to an ink jet recording head for performing the recording on a recording medium by discharging ink using an ink jet system.
2. Related Background Art
Regarding ink jet recording methods and recording heads, as disclosed in Japanese Patent Application Laid-open No. H04-010941 (1992), it is known to propose a technique in which a bubble generated by driving an electrothermal transducing element in response to a recording signal is communicated with the atmosphere. In this ink jet recording method, by stabilizing the volume of a flying ink droplet, a small amount ink droplet can be discharged at a high speed. Further, by eliminating cavitation generated during extinguishing the bubble, endurance of a heater can be enhanced. In the above-mentioned patent document, as an example of a design for communicating the bubble with the atmosphere, an arrangement in which a shortest distance between the electrothermal transducing element for generating the bubble in the ink and a discharge port as an opening through which the ink is discharged is greatly shortened in comparison with the prior art is proposed.
Now, the arrangement of the recording head of this type will be described. The recording head of this type comprises an element substrate on which the electrothermal transducing elements for causing the ink to be discharged are provided, and a flow path constituting substrate joined to the element substrate to form ink flow paths. The flow path constituting substrate defines a plurality of nozzles through which the ink flows, supply chambers for supplying the ink to the respective nozzles, and a plurality of discharge ports as nozzle tip end openings for discharging an ink droplet. The nozzle includes a bubble generating chamber within which the bubble is generated by the electrothermal transducing element, and a supply path for supplying the ink to the bubble generating chamber. On the element substrate, the electrothermal transducing elements are provided to be disposed in the respective bubble generating chambers. Further, in the element substrate, a supply port is provided for supplying the ink to the supply chambers from a back surface side opposite to a main surface of the element substrate which is contacted with the flow path constituting substrate. Further, the discharge ports are provided in a confronting relationship to the respective electrothermal transducing elements on the element substrate.
Further, in the recording head so designed, the ink supplied from the supply port to the supply chambers is supplied along the nozzles and is loaded within the bubble generating chambers. The ink loaded in the bubble generating chamber is discharged as an ink droplet in a direction substantially perpendicular to the main surface of the element substrate by the bubble generated by film boiling of the ink caused by the electrothermal transducing element.
By the way, in the above-mentioned recording head, when the ink droplet is discharged, the ink loaded in the bubble generating chamber is divided into a flow toward a discharge port side and a flow toward a supply path side by the bubble growing within the bubble generating chamber. In this case, pressure generated by the bubbling of the fluid may escape toward the supply path side or may be partially lost by the friction against an inner wall of the discharge port.
This phenomenon adversely influences the discharging and tends to become noticeable as the liquid droplet to be discharged becomes smaller. That is to say, if a diameter of the discharge port is reduced to form a smaller size liquid droplet, the resistance of the discharge port portion is greatly increased, and thus, since the flow rate toward the discharge port direction is increased and the flow rate toward the flow path direction is decreased, the discharging speed of the ink droplet is decreased.
In order to solve this problem, U.S. Patent Publication No. 2003-0016270 proposes a head as shown in FIGS. 9A to 9C. FIG. 9A is a plan view of the recording head, viewed from a direction perpendicular to a substrate, FIG. 9B is a sectional view taken along the line 9B-9B in FIG. 9A and FIG. 9C is a sectional view taken along the line 9C-9C in FIG. 9A.
The ink jet recording head shown in FIGS. 9A to 9C comprises a plurality of nozzles through which ink flows, supply chambers 106 for supplying ink to the respective nozzles, and a plurality of discharge ports 104 as nozzle tip end openings for discharging the ink droplet. The recording head is provided with a flow path constituting substrate, and an element substrate on which heaters as electrothermal transducing elements are provided and which has a main surface to which the flow path constituting substrate is joined. The flow path constituting substrate includes first discharge port portions including first discharge ports 104, bubbling chambers 111 within each of which a bubble is generated by the heater 101, second discharge port portions 110 communicating between the respective first discharge port portion 104 and the respective bubbling chamber 111, and supply paths 109 for supplying the ink to the respective bubbling chambers 111. The second discharge port portion 110 is a cylindrical or frusto-conical space centered on a vertical line extending from the center of the first discharge port portion 104 to the element substrate vertically and is communicated with the corresponding first discharge port portion 104 and the corresponding bubbling chamber 111. Further, in the plan view viewed from the direction perpendicular to the main surface of the element substrate, when looking at the direction perpendicular to the main surface of the element substrate, an outer periphery of the second discharge port portion 110 is positioned outside an outer periphery of the first discharge port portion 104 and is positioned inside an outer periphery of the bubbling chamber 111.
In the recording head having the above-mentioned construction, by providing the second discharge port portion 110 having a cross-sectional area greater than that of the first discharge port portion 104 in a direction perpendicular to an ink flowing direction, the total flow resistance of the ink flow flowing toward the discharge port 104 is reduced. Thus, since the bubble generated by the bubble generating action is growing toward the discharge port 104 with less pressure loss, the flow rate of the ink escaping toward the flow path can be suppressed and, thus, the reduction of the discharging speed of the ink droplet can be prevented.
However, if one tries to further reduce the dimension of the discharge liquid droplet, the diameter of the discharge port must be further decreased and, thus, the flow resistance toward the direction of the discharge port must be further decreased in comparison with the recording head having the above-mentioned second discharge port portion. To this end, the cross-sectional area of the second discharge port portion must be increased; in this case, however, when the bubble is generated in the ink by driving the heater, the ink is apt to be stagnated. As a result, the ink supplied from the supply path flows into the second discharge port portion and a meniscus in the discharge port is offset, with the result that the speed of the firstly flying main droplet becomes very slow and smaller diameter liquid droplets (satellite droplets) are discharged subsequently (refer to FIG. 5H).
Such satellite droplets may deteriorate an image if they are adhered to the recording medium and, if the satellite droplets do not reach the recording medium, they will constitute floating ink mist which may be adhered to various elements of a recording apparatus (printer) to contaminate the latter.
Further, other than the satellite droplets, when the main droplet is flying, due to the offset of the meniscus, the main droplet itself flies unstably, thereby deteriorating the recorded image.
Furthermore, when the discharging is performed continuously at high frequency, the stagnation of the ink in the second discharge port portion may cause dispersion in discharging volume due to heat accumulated in the ink.